The aim of this project is to characterize the SV40 large tumor (T) antigen found in association with the SV40 nucleoprotein complexes extracted from SV40-infected cells (Mann, K. and Hunter, T., J. Virol. 29 : 232 (1979)). It seems likely that this is a functional association involved in the initiation of SV40 DNA synthesis since SV40 T antigen is not detectable in nucleoprotein complexes from SV40 tsA-infected cells incubated at the nonpermissive temperature. We will, therefore, characterize the T antigen associated with the nucleoprotein complexes with respect to its chemical composition, its specific site of binding on the SV40 DNA, its relative association with SV40 (RI) DNA-containing complexes versus SV40 (I) DNA-containing complexes, the effect of salts and divalent cations on its association with the nucleoprotein complexes, its heat lability and ability to undergo renaturation, its ability to bind to nucleoprotein complexes in vitro, and its enzymatic activity. We will specifically examine modification (acetylation, glycosylation, and phosphorylation) of the nucleoprotein complex T antigen to see if it differs from the modification found in other populations of T antigen molecules, in particular in the T antigen molecules not associated with the nucleoprotein complexes. It is possible that modification influences the ability of large T antigen to bind to SV40 DNA. We will attempt to identify the enzymatic activity of the nucleoprotein complexes associated with and those not associated with the SV40 large T antigen. The enzymatic activity of large T antigen associated with the complexes may differ from the enzymatic activity of the T antigen molecules not bound to the DNA in the nucleoprotein complexes. With these studies, we hope to further our understanding of the biological role of SV40 large T antigen in the initiation of SV40 DNA synthesis and in cell transformation.